In recent years, in a welding step of welding automotive components, a resistance spot welding method is widely used in which a plurality of steel plates are overlapped and then sandwiched by a pair of electrodes, and the overlapped body is then energized while being pressed so as to form a molten pool (usually called “nugget”) at the steel plate interface and thereby to join the steel plates.
For example, a door opening portion of an automobile includes, as structural members, a pillar and a roof rail. A pillar 20 includes an overlapped body 21 in which, for example, three steel plates respectively constituting an outer steel plate, a reinforced steel plate and an inner steel plate are overlapped (hereinafter referred to as “steel plates” including the overlapped body). As shown in FIG. 4, the overlapped body 21 is joined by forming weld portions 23 at a predetermined interval by spot welding in a flange 22 of the overlapped body 21.
As the overlapped body described above, various combinations (hereinafter referred to as “plate combinations”) can be conceived by selecting the number of steel plates and the material of the steel plates, but for some plate combinations, it may be difficult to find appropriate welding conditions for obtaining a sound nugget (hereinafter, such combinations will be referred to as “difficult-to-weld plate combinations”). As used herein, “sound nugget” refers to a nugget having a sufficiently large molten pool and exhibiting a sufficient joint strength in a tensile test or the like using a joined body (joint). As the size of a weld portion, for example, generally, a value of 4√t or more (where t is the thickness (mm) of the thinner one of the steel plates forming the steel plate interface) is used as the reference. A typical example of a difficult-to-weld plate combination is an overlapped configuration of three steel plates including a thin steel plate, a thick steel plate 1 and a thick steel plate 2 as used in the door opening portion of the body of an automobile described above. Generally, for these members, a mild steel plate having a thickness of 1.0 mm or less is used as the thin steel plate, and high-tensile strength steel plates having a thickness of 1.2 mm or more and a tensile strength of 340 MPa or more are used as the thick steel plate 1 and the thick steel plate 2.
The reason that it is difficult to find appropriate welding conditions for obtaining a sound nugget for the difficult-to-weld plate combination will be described below.
Specifically, if the amount of energization is too large with respect to the contact electric resistance (hereinafter referred to simply as “contact resistance”) at an interface between steel plates that are in contact with each other, erosion called expulsion and surface flash (also called spatter, splash or the like) caused as a result of the steel plate interface being overheated due to resistive heating is likely to occur. If, on the other hand, the amount of energization is too small, the amount of resistive heating at the steel plate interface is reduced, and thus the molten pool cannot be made sufficiently large. In the case of, for example, the overlapped body composed of three steel plates, there are two steel plate interfaces, but the amount of resistive heating at a steel plate interface between the thin steel plate and the thick steel plate 1 is relatively smaller than the amount of resistive heating at a steel plate interface between the thick steel plate 1 and the thick steel plate 2. For this reason, if welding is performed at one of the steel plate interfaces by using the amount of energization that is preferable to obtain a sound nugget without causing the expulsion and surface flash, the amount of resistive heating at the other steel plate interface will inevitably be too large or small. As a result, the expulsion and surface flash will be generated at the interface between the thick steel plate 1 and the thick steel plate 2, or the nugget formed at the interface between the thin steel plate and the thick steel plate 1 will be insufficient. For this reason, generally, the amount of energization is set to be relatively high although the expulsion and surface flash occurs at the interface between the thick steel plate 1 and the thick steel plate 2.
Patent Document 1 discloses a method for making a spot welded joint including a nugget having a required size without causing the expulsion and surface flash in a difficult-to-weld plate combination as described above. According to the invention disclosed in Patent Document 1, a two-stage welding process including a first stage welding process and a second stage welding process is performed, wherein the second stage welding process is performed with a higher welding pressure, a lower current or the same current, and a longer or the same energization time with respect to the first stage welding process.
Other than the above, Patent Document 2 discloses a method for performing welding in a difficult-to-weld plate combination, wherein the welding pressure of an electrode chip on a workpiece is changed between the front surface and the back surface. Patent Document 3 discloses a method in which the cooling effect of cooling water that circulates through an electrode is alleviated by interposing a welding aid between the thin steel plate and the electrode.
In a welding step of welding automotive components or the like with the use of resistance spot welding, generally, a plurality of welding spots are provided consecutively at locations required from the design point of view. Accordingly, when resistance welding is performed at a given location, if there already is a welding spot near the location (hereinafter referred to as “existing welding spot”), a branch current that flows through the existing welding spot as an energization path is generated. Another case is also conceived in which, an energization path is formed at a location other than the existing welding spot and a branch current is generated depending on the geometric shape of members and the arrangement of space with another member. As described above, if the welding current is branched at the time of welding, formation of the molten pool is delayed and thus a sound nugget cannot be obtained. The branch current is also called reactive current, and various investigations have been made on the method for limiting the influence thereof.
For example, Patent Document 4 discloses an invention in which a reactive current is calculated and a current that is increased by an amount corresponding to the calculated reactive current is set as a welding current. Patent Document 5 discloses a method for obtaining a sound nugget by forming a slit so as to reduce the influence of reactive current. Patent Documents 6 and 7 disclose inventions in which an auxiliary electrode is provided near an electrode for the purpose of adjusting the branched state of the welding current so as to give a proper heating condition near the welding spot.